1. Field of the Invention
The invention relates generally to the field of the evaluation of wellbore casing. More specifically, the present invention relates to a method and apparatus to provide for the analysis of the bond that secures casing within a wellbore.
2. Description of Related Art
Hydrocarbon producing wellbores typically comprise tubulars, such as casing 8, set within the wellbore 5. Typically the casing 8 is bonded to the wellbore by adding cement 9 within an annulus formed between the outer diameter of the casing 8 and the inner diameter of the wellbore 5. The cement bond not only adheres the casing 8 within the wellbore 5, but also serves to isolate adjacent zones (Z1 and Z2) within the formation 18 from one another. Isolating adjacent zones can be important when one of the zones contains oil or gas and the other zone includes a non-hydrocarbon fluid such as water. Should the cement 9 surrounding the casing 8 be defective and fail to provide isolation of the adjacent zones, water or other undesirable fluid can migrate into the hydrocarbon-producing zone thus diluting or contaminating the hydrocarbons within the producing zone.
To detect possible defective cement bonds, downhole tools 14 have been developed for analyzing the integrity of the cement 9 bonding the casing 8 to the wellbore 5. These downhole tools 14 are lowered into the wellbore 5 by wireline 10 in combination with a pulley 12 and typically include transducers 16 disposed on their outer surface formed for acoustic coupling to the fluid in the borehole. These transducers 16 are generally capable of emitting acoustic waves into the casing 8 and recording the amplitude of the acoustic waves as they travel, or propagate, across the surface of the casing 8. Characteristics of the cement bond, such as its efficacy and integrity, can be determined by analyzing the attenuation of the acoustic wave.
One example of such an acoustic transducer is a piezoelectric device having a piezoelectric crystal that converts electrical energy into mechanical vibrations or oscillations. The vibrations produced by the piezoelectric device can be transmitted to the casing 8 that in turn form acoustic waves in the casing 8. The acoustic waves impart energy to the tubular member to create a frequency response from the tubular and monitored by receiver transducers. The measured response collected can be analyzed real-time or stored as data for subsequent analysis. The data can be in analog or digital format.